The following relates to the magnetic resonance arts. It finds particular application in high field magnetic resonance imaging, at, for example, approximately 3 Tesla or higher, and will be described with particular reference thereto. However, it also finds application in magnetic resonance imaging or spectroscopy performed at lower magnetic fields, and in the like applications which may benefit from a controlled B1 magnetic field.
In magnetic resonance imaging (MRI), an imaging subject is placed in a temporally constant main B0 magnetic field and subjected to radio frequency (RF) excitation pulses to generate nuclear magnetic resonances in the imaging subject. Magnetic field gradients are superimposed on the main B0 magnetic field to spatially encode the magnetic resonances. The spatially encoded magnetic resonances are read out and reconstructed based on the spatial encoding to generate magnetic resonance images.
Typically, RF coils are used for transmit and receive modes. In the transmit mode, RF coils generate a B1 magnetic field that excites nuclear spins from low-energy states to high-energy states at the corresponding Larmor Frequency. In the receive mode, the same set or a different set of RF coils detect the echo generated by nuclear spins that transit from high-energy states to low-energy states. In the transmit mode, RF coils are expected to provide the desired excitation, e.g., a B1 magnetic field profile for a given imaging method. However, at the higher main B0 magnetic fields, such as at approximately 3 Tesla or higher. For example, when the imaging is performed at 7 Tesla, the resonant or Larmor frequency of 1H shifts into the very high frequency (VHF) or ultra high frequency (UHF) domain. Electrodynamic material properties of the imaged subject, such as electric conductivity and dielectric permittivity increasingly distort the transmitted B1 magnetic field. These distortions are typically subject-dependent, and may also depend upon the positioning of the imaging subject, the region of interest and distribution of macroscopic fractions with different electrodynamic material properties within the subject that is being imaged. For example, dynamic reordering/redistribution of dielectric properties (heart/lung placement, -size, -shape) may occur which needs to be addressed within the whole body in vivo investigation.
At higher magnetic field strengths, the axial dimension of the region of interest (ROI) is comparable to or larger than a wavelength. The sinusoidal or the co-sinusoidal current distribution provided by the first Fourier mode does not generate a homogeneous field inside such a finite-length ROI. The phase variation in the transverse dimension becomes large and hot spots appear at the phantom center due to the so-called dielectric resonance effect.
Several methods have been proposed to improve high-field B1 magnetic field homogeneity. One approach seeks closer approximations of boundary current distributions with respect to a finite-length ROI. The approximation is implemented by distributed circuitry.
Another approach to improve the homogeneity of B1 magnetic field is to actively control the phase and magnitude of the transmit signal, for example, with a phased-array transmit coil. However, due to the axial invariance of most phased-array structures, it is typically found that B1 homogeneity may only be optimally achievable on one axial slice for one phase-magnitude configuration.
Another approach to improve the homogeneity of B1 magnetic field is to use shimming by inserting high-permittivity material. More specifically, for non traveling-wave coils, where subjects are treated as dielectric resonators, the equivalent ROI radius is increased by inserting high-permittivity material; thus, the B1 magnetic field homogeneity is accordingly improved.
Yet, some MRI applications require localized B1 magnetic field excitations. The localized B1 magnetic field excitations have the advantage of reduced specific absorption rate (SAR) and thus improved patient safety. For example, in some arterial spin labeled (ASL) perfusion MRI, RF coils are used to saturate the proton spins in the common carotid arterial. In in-vivo spectroscopic MR imaging, spins in a specific region are selectively excited.